The Global System for Mobile Communications (GSM: originally from Groupe Spécial Mobile) is currently the most popular standard for mobile phones in the world and is referred to as a 2G (second generation) system. W-CDMA (Wideband Code Division Multiple Access) is a type of 3G (third generation) cellular network. W-CDMA is the higher speed transmission protocol designed as a replacement for the aging 2G GSM networks deployed worldwide. More technically, W-CDMA is a wideband spread-spectrum mobile air interface that utilizes the direct sequence Code Division Multiple Access signaling method (or CDMA) to achieve higher speeds and support more users compared to the older TDMA (Time Division Multiple Access) signaling method of GSM networks.
The ability to perform handovers without interruption of service is a key requirement for all cellular networks. Historically, this has focused on supporting a voice call during handover for a given cellular technology. This situation has now changed with the need to support cell transitions for multiple services (voice, video and data) and also to seamlessly encompass a variety of wireless technologies (GSM/EDGE, WCDMA/HSDPA). There are two reasons why a handoff (handover) might be conducted: if the phone has moved out of range from one cell site (base station) and can get a better radio link from a stronger transmitter, or if one base station is full the connection can be transferred to another nearby base station.
The most basic form of handoff is that used in GSM and analog cellular networks, where a phone call in progress is redirected from one cell site and its transmit/receive frequency pair to another base station (or sector within the same cell) using a different frequency pair without interrupting the call. In GSM, the access technology is TDMA based and hence, the mobile's receiver can use “free timeslots” to change frequency and make measurements. Using the information from the measurement reports, the network can then choose to instruct the mobile to perform a handover from its existing serving cell to a given target cell as defined. As the phone can be connected to only one base station at a time and therefore needs to drop the radio link for a brief period of time before being connected to a different, stronger transmitter, this is referred to as a hard handoff. This type of handoff is described as “break before make” (referring to the radio link).
In CDMA systems the phone can be connected to several cell sites simultaneously, combining the signaling from nearby transmitters into one signal using a rake receiver. Each cell is made up of one to three (or more) sectors of coverage, produced by a cell site's independent transmitters outputting through antennas pointed in different directions. The set of sectors the phone is currently linked to is referred to as the “active set”. A soft handoff occurs when a CDMA phone adds a new sufficiently-strong sector to its active set. It is so called because the radio link with the previous sector(s) is not broken before a link is established with a new sector; this type of handoff is described as “make before break”. In the case where two sectors in the active set are transmitted from the same cell site, they are said to be in softer handoff with each other.
There are also inter-radio access technology (I-RAT) handoffs where a call's connection is transferred from one access technology to another, e.g. a call being transferred from GSM to W-CDMA. If the mobile phone leaves a cell and no new cell can be found in the same system, the base station can hand over an appropriately equipped mobile phone to a cell in another system. These intersystem handovers are highly complex because two technically disparate systems must be combined with each other. Basically, there are two handover options from WCDMA to GSM: In the case of blind handover, the base station simply transmits the mobile phone with all relevant parameters to the new cell. The mobile phone changes “blindly” to the GSM cell, i.e. it has not yet received any information about the timing there. It will first contact the transmitted control channel (BCCH), where it tries to achieve the frequency and time synchronization within 800 ms. Next, it will switch to the handed-over physical voice channel, where it will carry out the same sequence as with the non-synchronized intercell handover. For the second type of handover from WCDMA to GSM, the compressed mode is used within the WCDMA cell; in this mode, transmission and reception gaps occur during the transmission between base station and mobile phone. During these gaps, the mobile phone can measure and analyze the nearby GSM cells. For this purpose, the base station, similar to the GSM system, provides a neighbor cell list and the mobile phone transfers the measurement results to the base station. The actual handover in the compressed mode is basically analogous to blind handover.
During handovers, muting of the audio provided to the cell phone user is generally required in order to compensate for the interruptions to the data stream.